This invention relates to a method of forming, on a heat-resistant substrate, a superconductive thin film having high onset transition temperature and also a narrow temperature range in which transition to superconductor occurs, which is suitably used as a magnetic coil material in fusion reactors, magnetohydrodynamic generators, accelerators, rotary electric appliances, magnetic separators, linear motor cars, nuclear magnetic resonance devices, magnetic propulsion ships, various laboratory equipments, etc.; as a power transmission material for transmission cables, energy storage device, transformers, rectifiers, phase modifiers, etc., where electrical loss is a serious factor; further as various elements such as a Josephson element, a SQUID element, etc.; and also as various sensors such as a magnetic sensor, IR sensor, etc., and to a solution for forming such superconductive thin film.
In recent years, it is practiced to form a thin film of a superconductive material comprising an oxide of a ceramic which can exhibit excellent superconductive characteristics on the surfaces of various types of substrates.
As the method of forming a thin film of a superconductive material comprising an oxide of a ceramic on the surface of a substrate, known are sputtering and screen printing.
On the former sputtering method, the following studies are disclosed: "Compositional and Structural Analyses for Optimizing the Preparation Conditions of superconducting (La.sub.1-x Sr.sub.x CuO.sub.4-.delta.) Films by Sputtering", Japanese Journal of Applied Physics, Vol. 26, No. 4, April, 1987, pp L388-L390, M. Kawasaki et al.; "High Tc Thin Films of (La.sub.1-x M.sub.x).sub.y CuO.sub.4-.delta. (M=Sr, Ba, Ca) Prepared by Sputtering", ibid.; Vol. 26, No. 4, April, 1987, pp. L410-L412, S. Nagata et al.; "Deposition of Sr.sub.x La.sub.2-x CuO.sub.4-y Thin Films by Sputtering", ibid.; Vol. 26, No. 4, April, 1987, pp. L508-L509, N. Terada et al.; "Josephson Junctions Observed in La.sub.1.8 Sr.sub.0.2 CuO.sub.4 superconducting Polycrystalline Films", ibid.; Vol. 26, No. 4, April, 1987, pp. L521-L523, K. Moriwaki et al.; "Hall Effect in superconducting (La.sub.1-x Sr.sub.x).sub.2 CuO.sub.4 Single Crystal Thin Films", ibid.; Vol. 26, No. 4, April, 1987, pp. L524-L525, M. Suzuki et al.; "Preparation and Characterization of superconducting Y-Ba-Cu-O Thin Films", ibid.; Vol. 26, No. 5, May 1987, pp. L709-L710, H. Adachi; "Meissner Effect in High-Tc Superconductive Thin Films", ibid.; Vol. 26, No. 5, May, 1987, pp. L724-L725, S. Hatta et al.; "High To Yb-Ba-Cu-O Thin Films Deposited on Sintered YSZ Substrates by Sputtering", ibid.; Vol. 26, No. 5, May, 1987, pp. L738-L740, M. Kawasaki et al., etc.
On the latter screen printing method, following studies are disclosed: "Preparation of (La.sub.1-x Sr.sub.x).sub.2 CuO.sub.4-.delta. superconducting Films by Screen Printing Method", Japanese Journal of Applied Physics; Vol. 26, No. 4, April, 1987, pp. L399-L401, H. Koinuma et al.; "High-Tc Superconductivity in Screen Printed Yb-Ba-Cu-O Films", ibid.; vol. 26, No. 5, May, 1987, pp. L761-L762, H. Koinuma et al., etc.
However, in the former sputtering method, when a superconductive thin film having a homogeneous multi-component composition is to be formed on the surface of a substrate according to the above method, it is difficult to form a thin film as a single phase having the desired composition, and phases of undesired compositions may also be present, resulting in poor superconductivity.
On the other hand, the latter screen printing method, where a film is formed by coating the surface of a substrate with a paste containing particles of oxides, followed by calcination, involves a problem that a thin film having a homogeneous composition may not be obtained if the particles do not have a homogeneous composition to be liable to make the film thickness nonuniform, and further cracks and the like may readily be formed on the film.